It is hard to tell from limited info you provided. Make sure your device is added to the Facebook developer program to have test advertisements pushed to it. I would say you should ask Facebook support on this one.
Edit: I see you added additional details. Can you explain what a “link” is, where it is shown and where it leads the user?
Thank you for clarification. Make sure you are accessing the app on iPhone from the same Facebook account you are accessing it from Android. I think there was an option to add accounts as testers, so they may access the app which is not yet published.
If both Android and iPhone use the same account - you should ask for support from Facebook.
Thank you very much for your response mate. I am using the same account on both Android phone and iPhone. But iPhone is neither taking me anywhere nor showing any error logs. I ll contact support if nothing works. Thanks again